Multiple Input Multiple Output (MIMO) technology where wireless transmission is performed using multiple transmission/reception antennas is gathering attention as a technology which can greatly improve frequency usage efficiency, and is in practical use in cellular systems and wireless LAN systems and so forth. The amount of improvement in frequency usage efficiency by the MIMO technology is proportionate to the number of transmission/reception antennas. However, there is a limit to the number of transmission/reception antennas which can be provided to a terminal device serving as a reception device. Now, a downlink multi-user MIMO (MU-MIMO) where multiple terminal devices connected at the same time are handled as a virtual large-scale antenna array, and transmission signals from a base station device (transmission device) to the terminal devices are spatially multiplexed, is effective in improving frequency usage efficiency.
The spatially-multiplexed transmission signals addressed to the terminal devices (users) are received at the terminal devices as inter-user interference (IUI), resulting in marked property deterioration if nothing is done. There have been several methods proposed (NPL 1) where transmission signals can be generated with suppressed IUI at the time of reception by the terminal devices, without placing a great load on the terminal devices, if the base station device knows the state of the propagation channels from the transmission antennas of the base station device to the reception antennas of the terminal devices.
For example, there is a method where the transmission signals are subjected to precoding at the base station device before transmission, so as to be received at the terminals devices in a state where IUI has been suppressed. An example thereof is linear precoding (LP) where transmission signals are precoded by linear processing. An example of linear precoding is zero-forcing (ZF) precoding in which the transmission signals are weighted (the transmission signals are multiplied by W=H−1) where an inverse matrix H−1 (or pseudo inverse matrix Ht=HH(HHH)−1 where the superscript H represents the Hermitian conjugate) is used as a weighting matrix (linear filter) W, obtained from a propagation channel matrix having complex propagation channel gains among the transmission antennas and the reception antennas of the terminal devices obtained from information representing the state of propagation channels (channel state information: CSI) as its elements. Another example of linear precoding is minimum mean square error (MMSE) precoding where transmission signals are weighted by a weighting matrix (linear filter) W=HH(HHH+αI)−1 (where I represents a unit matrix and α represents a normalization coefficient). Linear precoding (linear beam forming) where the base station multiples the transmission signals beforehand by a linear filter, calculated based on propagation channel information notified from the terminal devices, to suppress IUI, is employed in cellular systems such as LTE (Long Term Evolution) and LTE-Advanced, and wireless LAN systems such as IEEE 802.11ac and so forth, for example.
On the other hand, wireless LAN has rapidly come into widespread used in recent years, due to increased demand for high-speed wireless communication networks in offices and homes. Next-generation LAN is being drawn up by the TGac of IEEE (The Institute of Electrical and Electronic Engineers, Inc.) 802.11, and extending the channel frequency band beyond 80 MHz and introducing the above-described MU-MIMO is being studied (NPL 2), aiming for realization of throughput of 1 Gbps or higher.